A well capable of producing oil or gas will have a conductor housing secured to a string of conductor pipe, the conductor pipe extending a short depth into the well. A wellhead housing is then landed in the conductor housing and secured to an outer or first string of casing. The first string of casing extends through the conductor to a deeper depth into the well. Depending on the particular conditions of the geological strata above the target zone (typically, either an oil or gas producing zone or a fluid injection zone), one or more additional casing strings will extend through the outer string of casing to increasing depths until the well is cased to its final depth. The outer strings of casing are supported at the upper end by their own casing hangers that are landed in and supported by the wellhead housing. Between each casing hanger and the wellhead housing, a casing hanger seal assembly is set to isolate each annular space between strings of casing. The last, and innermost, string of casing, the production casing, extends into the well to the final depth.
After drilling and installing the casing strings is complete, the well must be completed for production of the production fluids. The production fluids typically flow from the earth formation through perforations made in the production casing at the producing zone. A string of production tubing extends to the producing zone within the production casing to provide a pressure-controlled conduit through which the well fluids are produced. At some point above the producing zone, a packer seals the space between the production casing and the production tubing to ensure that the well fluids flow through the production tubing to the surface. The tubing is supported by a tubing hanger assembly that lands and locks above the production casing hanger. A tubing hanger seal assembly is set to isolate the annular space between the production tubing and the production casing.
At the wellhead, various arrangements of production control valves are arranged in an assembly generally known as a tree installed on the wellhead housing. The tree has one or more production bores containing actuated valves and extending to respective production fluid outlets.
For example, a tree with the arrangement of production control valves offset from the production tubing, generally called horizontal trees, can be used. One type of horizontal tree is a Spool Tree™ shown and described in U.S. Pat. No. 5,544,707, hereby incorporated herein by reference. A horizontal tree also locks and seals onto the wellhead housing. In horizontal trees, however, the tubing hanger locks and seals in the tree bore. With the production valves offset from the production tubing, the production tubing hanger and production tubing may be removed from the tree without having to remove the horizontal tree from the wellhead housing.
The production tubing hanger assembly locks and seals in the drill-through horizontal tree instead of in the wellhead housing. However, the annular space where the tubing hanger seal assembly resides does not always remain constant as pressure and thermal expansion and external forces act on the elements that bound the annular space. Also asymmetric geometry of the members that form the bore or the mandrel can cause asymmetric deflections as loads are applied to the members. The tubing hanger seal assembly should be compliant in the radial direction to conform to these movements of the seal surfaces and maintain sufficient contact pressures with the seal surfaces to maintain a seal for all conditions.